The present invention relates to a method and device for monitoring the maximum distance between two objects, in particular, between a child and its guardian, with the help of two transceivers, of which the first periodically transmits status messages of a certain transmission power to the second, wherein an alarm is triggered in the second transceiver when the reception of the status messages decreases, and wherein information on the transmission power is also transmitted in each status message.
A method and a device of this type are known from U.S. Pat. No. 5,963,130. Because the communications between the transceivers is limited to periodic messages with intermediate pauses, energy consumption reduces. U.S. Pat. No. 5,963,130 shows, in detail, a child-monitoring device wherein the parent station evaluates the reception field strength of the polling reply of the child station and compares it with a threshold. When the value falls below the threshold, a power control command is integrated in the next polling query and transmitted to the child station. The child station then increases its reply transmission power and integrates into its reply message performance information that it is now transmitting at high power. The transmission power specified by the child station in its reply is used to trigger the range alarm: if the reply had been transmitted with high transmission power, the alarm is immediately triggered. The performance control command transmitted from the parent station to the child station thus always depends on only the reception field strength and its comparison with the fixed threshold. The principle of such a reception field strength evaluation does not allow detecting whether a weak reception field strength is to be traced to low transmission power or to a large distance.
For everyday, practical use of an object range monitoring device, in particular, for monitoring children for unintentional separation, running off, kidnapping, etc., a robust detector of when a maximum distance has been exceeded that is not susceptible to noise under any circumstances is indispensable, and all of this for minimal energy consumption. The invention sets the goal of creating a method and a device of the type mentioned above that satisfies these requirements.